3.1. Hematopoietic Cell Production
The morphologically recognizable and functionally capable cells circulating in blood include erythrocytes, neutrophilic, eosinophilic, and basophilic granulocytes, B-, T-, non B-, non T-lymphocytes, and platelets. These mature hematopoietic cells derive from and are replaced, on demand, by morphologically recognizable dividing precursor cells for the respective lineages such as erythroblasts for the erythrocyte series, myeloblasts, promyelocytes and myelocytes for the granulocyte series, and megakaryocytes for the platelets. The precursor cells derive from more primitive cells that can simplistically be divided into two major subgroups: stem cells and progenitor cells (for review, see Broxmeyer, H. E., 1983, "Colony Assays of Hematopoietic Progenitor Cells and Correlations to Clinical Situations," CRC Critical Reviews in Oncology/Hematology 1:227-257). The definitions of stem and progenitor cells are operational and depend on functional, rather than on morphological, criteria. Stem cells have extensive self-renewal or self-maintenance capacity (Lajtha, L. G., 1979, Differentiation 14:23), a necessity since absence or depletion of these cells could result in the complete depletion of one or more cell lineages, events that would lead within a short time to disease and death. Some of the stem cells differentiate upon need, but some stem cells or their daughter cells produce other stem cells to maintain the precious pool of these cells. Thus, in addition to maintaining their own kind, pluripotential stem cells are capable of differentiation into several sub-lines of progenitor cells with more limited self-renewal capacity or no self-renewal capacity. These progenitor cells ultimately give rise to the morphologically recognizable precursor cells. The progenitor cells are capable of proliferating and differentiating along one, or more than one, of the myeloid differentiation pathways (Lajtha, L. G. (Rapporteur), 1979, Blood Cells 5:447).
A variety of infectious agents, genetic abnormalities and environmental factors can cause a deficiency in one or more hematopoietic cell types. For example, hematological abnormalities have been observed in HIV-1 infected individuals (the human immunodeficiency virus (HIV) has been implicated as the primary cause of the slowly degenerative immune system disease termed acquired immune deficiency syndrome (AIDS) (Barre-Sinoussi, F., et al., 1983, Science 220:868-870; Gallo, R., et al., 1984, Science 224:500-503)) particularly in the late stages of disease (Lunardi-Iskandar, Y. et al., 1989, J. Clin. Invest 83:610-615). These abnormalities include a reduction in CD4.sup.+ T cells as well as cytopenias of one or more hematopoietic lineages, often associated with bone marrow morphologic abnormalities and deficient progenitor cell growth (Lunardi-Iskandar, Y. et al., 1989, J. Clin. Invest 83:610-615; Louache, F. et al., 1992, Blood 180:2991-2999). Idiopathic thrombocytopenic purpura (ITP), characterized by significant reduction in platelet numbers, often afflicts subjects infected with HIV (Ballem, P. J. et al., 1992, N. Engl. J. Med. 327:1779). The destruction of platelets in sufferers of ITP appears to be mediated by platelet associated autoantibodies (Berchtold, P. and Wenger, M., 1993, Blood 81:1246; Ballem, P. J. et al., 1987, J. Clin. Invest. 80:33). Thus, because management of ITP generally involves immunosuppression, treatment of ITP in HIV infected patients is complicated as administration of immunosuppressive drugs is extremely detrimental in HIV infection.
Additionally, chemotherapy and radiation therapy used in the treatment of cancer and certain immunological disorders can cause pancytopenias or combinations of anemia, neutropenia and thrombocytopenia. Thus, the increase or replacement of hematopoietic cells is often crucial to the success of such treatments. (For a general discussion of hematological disorders and their causes, see, e.g., "Hematology" in Scientific American Medicine, E. Rubenstein and D. Federman, eds., Volume 2, chapter 5, Scientific American, New York (1996)).
Furthermore, aplastic anemia presents a serious clinical condition as the overall mortality of all patients with aplastic anemias, in the absence of stem cell therapy, is high. Approximately 60-75% of individuals suffering from the disorder die within 12 months, in the absence of new stem cells. The overall incidence of these diseases is approximately 25 new cases per million persons per year. Although it is extremely unlikely that a single pathogenic mechanism accounts for all aplastic anemias, it is clear that provision of new hematopoietic stem cells is usually sufficient to allow permanent recovery, since transplantation of patients with aplastic anemia with bone marrow obtained from identical twins (i.e., syngeneic) (Pillow, R. P., et al., 1966, N. Engl. J. Med. 275:94-97) or from HLA-identical siblings (i.e., allogeneic) (Thomas, E. D., et al., Feb. 5, 1972, The Lancet, pp. 284-289) can fully correct the disease. However, some patients with aplastic anemia reject the transplanted marrow. This complication is particularly common among patients who have been immunologically sensitized as a result of multiple therapeutic blood transfusions.
The current therapy available for many hematological disorders as well as the destruction of the endogenous hematopoietic cells caused by chemotherapy or radiotherapy is bone marrow transplantation. However, use of bone marrow transplantation is severely restricted since it is extremely rare to have perfectly matched (genetically identical) donors, except in cases where an identical twin is available or where bone marrow cells of a patient in remission are stored in a viable frozen state. Except in such autologous cases, there is an inevitable genetic mismatch of some degree, which entails serious and sometimes lethal complications. These complications are two-fold. First, the patient is usually immunologically incapacitated by drugs beforehand, in order to avoid immune rejection of the foreign bone marrow cells (host versus graft reaction). Second, when and if the donated bone marrow cells become established, they can attack the patient (graft versus host disease), who is recognized as foreign. Even with closely matched family donors, these complications of partial mismatching are the cause of substantial mortality and morbidity directly due to bone marrow transplantation from a genetically different individual.
Peripheral blood has also been investigated as a source of stem cells for hematopoietic reconstitution (Nothdurtt, W., et al., 1977, Scand. J. Haematol. 19:470-481; Sarpel, S. C., et al., 1979, Exp. Hematol. 7:113-120; Ragharachar, A., et al., 1983, J. Cell. Biochem. Suppl. 7A:78; Juttner, C. A., et al., 1985, Brit. J. Haematol. 61:739-745; Abrams, R. A., et al., 1983, J. Cell. Biochem. Suppl. 7A:53; Prummer, O., et al., 1985, Exp. Hematol. 13:891-898). In some studies, promising results have been obtained for patients with various leukemias (Reiffers, J., et al., 1986, Exp. Hematol. 14:312-315; Goldman, J. M., et al., 1980, Br. J. Haematol. 45:223-231; Tilly, H., et al., Jul. 19, 1986, The Lancet, pp. 154-155; see also To, L. B. and Juttner, C. A., 1987, Brit. J. Haematol. 66: 285-288, and references cited therein); and with lymphoma (Korbling, M., et al., 1986, Blood 67:529-532). Other studies using peripheral blood, however, have failed to effect reconstitution (Hershko, C., et al., 1979, The Lancet 1:945-947; Ochs, H. D., et al., 1981, Pediatr. Res. 15:601). Studies have also investigated the use of fetal liver cell transplantation (Cain, G. R., et al., 1986, Transplantation 41:32-25; Ochs, H. D., et al., 1981, Pediatr. Res. 15:601; Paige, C. J., et al., 1981, J. Exp. Med. 153:154-165; Touraine, J. L., 1980, Excerpta Med. 514:277; Touraine, J. L., 1983, Birth Defects 19:139; see also Good, R. A., et al., 1983, Cellular Immunol. 82:44-45 and references cited therein) or neonatal spleen cell transplantation (Yunis, E. J., et al., 1974, Proc. Natl. Acad. Sci. U.S.A. 72:4100) as stem cell sources for hematopoietic reconstitution. Cells of neonatal thymus have also been transplanted in immune reconstitution experiments (Vickery, A. C., et al., 1983, J. Parasitol. 69(3):478-485; Hirokawa, K., et al., 1982, Clin. Immunol. Immunopathol. 22:297-304).
Clearly, there is a tremendous need for methods of expanding blood cells in vitro or therapies which increase the production of hematopoietic cells in vivo.